Circuit for operating electric discharge devices



Se t. 8, 1953 E. A. H. FRENCH CIRCUIT FOR OPERATING ELECTRIC DISCHARGE DEVICES Filed Aug. 2, 1947 INVN TOR 52 1C 727-14012 HOWARD Frame-,

ATTORNEY Patented Sept. 8, 1953 OFFICE CIRCUIT FOR OPERATING ELECTRIC DISCHARGE DEVICES Eric Arthur Howard French, Christchurch, England Application August 2, 1947, Serial No. 765,713 In Great Britain April 21, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires April 21, 1964 12 Claims. 1

The present invention relates to circuits for operating an electric discharge device of the kind wherein the voltage needed to start or restart the discharge greatly exceeds the open circuit voltage of the supply from which the device is fed. More particularly the invention is concerned with a circuit for operating a high pressure mercury vapour lamp.

In order to initiate the discharge in air-cooled mercury vapour lamps in their cold condition it is usual to apply a high voltage, such as that obtained from a Tesla coil, across the electrodes. This causes a glow discharge to take place and so enable the main discharge to be initiated. Such a high voltage discharge may fail to strike the lamp when it is in the hot condition and the pressure of the mercury va our is high. Moreover, with lamps of high operational wattages with relatively large electrode spacing such a discharge as that produced by 3, Tesla coil is not capable of striking the lamp, even in the cold condition when the pressure of the filling (comprising the mercury vapour and a small amount of a rare gas) is only a few millimetres.

condition by the discharge from a Tesla coil, the lamp may be required to be brought into operation soon after switching off, before the temperature has fallen and while the pressure of mercury vapour is still high, so that the Tesla coil cannot be employed to strike it. Such an operational requirement would exist in the case of search-li hts employing mercury vapour lamps which have to be switched on rapidly soon after being extinguished.

In a practical example of a mercury vapour lamp employed in a searchlight, the lamp when running normally from, say, a 110 volt D. C. source passes a current of at least 140 amps. and

dissipates a total power of at least 11.5 kilowatts.

The pressure of the filling when cold is only a few millimetres but when the lamp has attained its normal running temperature the pressure is about fifteen atmospheres.

The present invention provides a method by which a mercury vapour lamp is started, either in the hot or cold condition, by means of a high voltage pulse capable of causing a high voltage to occur between the lamp electrodes. According to the invention, an operating circuit for a high pressure mercury vapour lamp comprises connections for applying the normal running voltage to the lamp electrodes, a resonant circuit connected to the electrodes and means for generating and applying the high voltage pulse Even if the lamp is capable of being struck in the cold 2 across the resonant circuit and electrodes, the decrement of the resonant circuit being such that the spark will persist for sufficient time to enable the main discharge to establish itself.

In the particular case of the lamp previously referred to, a voltage impulse with a peak value of approximately 20 kv. is required when the lamp is in the hot condition to break down the interelectrode gap of the lamp and for the resulting high voltage are to be instrumental in striking the lamp a maximum current, in the arc, of about 8 amps. is required initially. This impulse of current is produced by the discharge of a large condenser. However if the arc was produced by the direct discharge of such a charged condenser through the lamp, the duration of the arc would be no longer than a few micro-seconds, and the main low voltage, high current discharge would have no opportunity to establish itself before the end of the arc and therefore the lamp would merely flash and go out again. It is found that with the lamp considered, it is necessary for the arc to be maintained for about 35 micro-seconds to ensure that the lamp is properly struck, whatever its conditions of temperature and pressure. This is achieved by employing an oscillatory discharge having a frequency of about 345 kc./s. The max imum voltage across the lamp and the maximum current in the are, at the commencement of the discharge, were approximately 20 kv. and 8 amps, respectively, and the decrement of the circuit was such that the peak oscillatory voltage across the lamp had fallen to a few thousand volts after about 35 micro-seconds, the are discharge still persisting.

The invention can be put into practice in various ways, and one preferred arrangement will now be described by way of example with reference to the accompanying diagrammatic drawing which shows a circuit diagram of the starting and running arrangements of a high pressure mercury vapour lamp intended for use in a searchlight.

The mercury vapour lamp A has the main 110 volt D. C. supply connected across it via the contacts of a main contactor O and an H. F. choke N. Also connected across the lamp is the circuit for generating the surge of high frequency, comprising the condenser C1, transformer T, low capacity chokes L1 and L2, condenser C2, spark gap G and inductance L3.

The mercury switch D which controls the charge and discharge of condenser 01 comprises two sets of contacts S1 and S2 and is mounted upon a plate capable of partial rotation and pivoted about its lowest corner. The switch is operated to charge or discharge C1 by partial rotation of the plate caused by an iron armature EA, secured to the plate, which is mounted between the poles of an electromagnet E having a current limiting resistance R1 in series with its.

winding. The switch is normally held in one position by means of a spring P, energisation of the electromagnet causing it to assume the alternative position.

Relay F incorporated in the control circuit is magnetically operated and is of the type in which on successive energizations of an operating circuit, a circuit alternately is made and broken be tween a pair of contacts, the condition set up at one energization remaining until the. next. The operating circuit is connected between the terminals T1 and the contacts are connected one to each of the terminals T2. A circuit is thus made and broken between the terminals T2 on alternate energizations of the operating circuit by application of a voltage across the terminals T1. Relays of this type are described in British patent specification No. 257,632. A 4 pi. condenser C4 is connected across the terminals T2 to prevent sparking when the contacts open.

The push button control switch B comprises two. pairs of contacts a, b and c, d which are bridged in succession by sliding bridge members carried by a moving member of insulating material. The moving bridge members are separated: by a light spring so that on depressing the button, contacs a and b are first connected and are followed by the connection of contacts and (1. Similarly on releasing the button, contacts c and dare opened first.

The sequence of eventsv occurring during the switching on and off of the lamp will now be described. Operation of the press button switch. first closes contacts a and b and thus relay is energised by its coil which is, connected in series with limiting resistance R2. across the 110 volt supply. The circuit between the terminals T2 of relay F is closed and energizes the operating coil of; the main contactor O which closes and places the 110, volt Supply across the lamp in series with H. F. choke N.

In order to obtain the necessary inductance in a relatively small space, the. choke N' is wound with relatively fine wire which would not be capable of carrying thev main discharge current of the lamp. In series with the choke is the operating coil M of an electromagnetic switch S; which, when the main, discharge current flows through the lamp, closes its contacts and shortcircuits the choke N.

Continued pressure on the press button causes contacts 0 and d to be closed with the result that operating coil E is energised and the switch D is operated to close the contacts S1, the contacts S being opened.

The closing of S1 results in condenser G1, which is of 5000 ,uf. capacity, being charged up from the 110 volt supply through the current limiting resistance R3.

On releasing. the push button contacts, c and d are first opened and the operating coil E is deenergised with the result thatthe mercury switch changes over under the influence of its return spring, the contacts S2 being closed and the contacts S1 opened so that the charged condenser C1 is connected directly across the low impedance primary winding of the transformer T, through which flows as a result a very heavy surge of current, with a peak value which may reach 2500 amps. This heavy current surge initiates the events causing the striking of the lamp, to be described later.

Consider nowv the conditions which exist after the push button has been released and the lamp has struck. The circuit between the terminals T: of relay F is closed and the contacts S2 are held closed by the return spring of the switch D. If now the. push button is operated for a second time, the contacts a and b which close first, energise. the coil of relay F and now cause this relay to release, so breaking the circuit between the terminals T2, and de-energise the main contactor O- which releases and disconnects the volt supply from the lamp and charging circuit. Thus no charge is given to condenser C1 and the switch 1) is not operated when the contacts 0 and (1 close at the end of the push button travel. Also the lamp A, if it is running, will be extinguished. Release of the push button and deenergisation of relay F has no further effect.

The transient surge through the primary of T, referred to above, is stepped up by the action of the transformer and a voltage impulse of about 26 kv. in magnitude appears across the secondary winding. The condenser C2 of capacity .007 MI is thus charged up to this value through chokes L1 and L2. When the voltage across. C2 reaches a critical value (about 20 kv.) the spark gap G breaks down and the lamp Aand the inductance L3 in series are connected across the charged condenser C2. The interelectrode space of the lamp then breaks down and a spark discharge passes. Condenser C2 is then effectively discharging through the inductance L2 connected in series therewith. The values of inductance L3 and capacity C2 are such that the discharge is oscillatory in nature, having a frequency of approximately 345 kc ./s. The. arc discharge across the lamp electrodes is also of the same form, the peak values of voltage and current reached at the commencement of the train of oscillation, being about 20. kv. and 8 amps. respectively. The damping of the circuit is such that after about 35. ,usecs, the peak amplitude of the oscillation will have fallen to a few thousand volts. The high frequency oscillation which occurs during the discharge of condenser. 02 is prevented from passing. back through the transformer secondary winding by virtue of the two chokes L1 and L2. Similarly the main supply which is connected across thelampis prevented from short circuiting the high. frequency oscillation by virtue of the choke N. The condenser C5 and rectifier X are included to protect the main supply circuit from anyv small leakage of high frequency current which may unavoidably pass the choke N.

At some time after the high frequency oscillatory discharge has commenced, the main mercury vapour arc discharge will be struck and maintained the. high frequency are then being extinguished. When, the main discharge. current of. the lamp flows through the operating coil M of the switch S3 the choke N is short-circuited so that it has not to carry the main current except for the brief period necessary to effect operation of the. short circuiting, switch. The choke N has not therefore to be designed to carry the full load current of the lamp for any extended period.

I claim:

1. operating circuit for a high pressure vapor electric discharge lamp adapted to operate normally from adirect current source and having characteristics such that on starting operation with any combination of internal temperature and pressure, which may occur, a high frequency discharge must be maintained between the electrodes of the lamp for a predetermined minimum time before the normal direct current discharge can start and be maintained, said circuit comprising: a first pair of terminals for connection to the electrodes of the lamp; a resonant circuit electrically connected across said first pair of terminals; a high voltage pulse generator; a switching means connected in circuit with said pulse generator and causing a substantially unidirectional high voltage pulse to be generated on each occasion on which it is operated; means for applying the output of said pulse generator to the resonant circuit, the resonant circuit having a decrement and resonant frequency such that on application of a pulse from the said pulse generator when the lamp is connected to the first pair of terminals, a high frequency discharge will be maintained in the lamp for at least said predetermined minimum time; a second pair of terminals for connection to the direct current source for use in normal operation of the lamp; and circuit means completed on operation of said switching means for electrically connecting said second pair of terminals to said first pair of terminals.

2. An operating circuit for a high pressure vapor discharge lamp according to claim 1 in which said circuit means includes at least one 1 high frequency choke and relay means having a winding connected in series with the choke and a contact operated when the current in said winding reaches a value slightly less than the normal discharge current of the lamp, to short circuit said choke.

3. An operating circuit for a high pressure vapor electric discharge lamp adapted to operate normally from a direct current source and having characteristics such that on starting operation with any combination of internal temperature and pressure, which may occur, a high frequency discharge must be maintained between the electrodes of the lamp for a predetermined minimum time before the normal direct current discharge can start and be maintained, said circuit comprising: a first pair of terminals for connection to the electrodes of the lamp; a resonant circuit electrically connected across said first pair of terminals; a second pair of terminals for connection to a direct current source; a capacitance; a voltage step-up pulse transformer having a primary and a secondary winding; a switching means, said switching means being connected in circuit with said second pair of terminals, said capacitance and said primary winding, for sequentially on operation, connecting said capacitance across said second pair of terminals to charge substantially to the voltage of the source connected thereto, disconnecting said capacitance from said second pair of terminals without discharging it, and connecting said capacitance across the primary winding to discharge through it; means connecting the secondary winding across part at least of the resonant circuit, the resonant circuit having a decrement and resonant frequency such that on application of a substantialy unidirectional high poltage pulse thereto on discharge of said capacitance through the primary winding when the lamp is connected to said first pair of terminals, a high frequency discharge will be maintained in the lamp for at least said predetermined minimum time; a third pair of terminals for connection to the direct current source for use in normal operation of the lamp; and circuit means completed on operation of said switching means for electrically connecting said third pair of terminals to said first pair.

4. An operating circuit for a high pressure vapor discharge lamp according to claim 3 in which said circuit means includes at least one high frequency choke and relay means having a winding connected in series with the choke and a contact operated when the current in said winding reaches a value slightly less than the normal discharge current of the lamp, to short circuit said choke.

5. An operating circuit for a high pressure vapor electric discharge lamp according to claim 3 in which said switching means comprises: a push button switch having a first and a second pair of contacts which are closed on operation in that order and open in that order on release; a first relay means connected in circuit with said first pair of contacts across said third pair of terminals for completing and breaking the connection between said third and first pairs of terminals of alternate occasions of closing said first pair of contacts; a second relay means and a changeover switch, said second relay means being connected in circuit with said second pair of contacts across said third pair of terminals, changing the switch from a first to a second position on energization and back on de-energization and said changeover switch in said first position connecting the capacitance across the said third pair of terminals and in said second position connecting the capacitance across the said primary winding.

6. An operating circuit for a high pressure vapor electric discharge lamp adapted to operate normally from a direct current source and having characteristics such that on starting operation with any combination of internal temperature and pressure, which may occur, a high frequency discharge must be maintained between the electrodes of the lamp for a predetermined minimum time before the normal direct current discharge can start and be maintained, said circuit comprising: a first pair of terminals for connection to the electrodes of the lamp; a resonant circuit electrically connected across said first pair of terminals, said resonant circuit ineluding a capacitanpe, an inductance and a spark gap connected in series; a second pair of terminals for connection to a direct current supply; a further capacitance; a voltage step-up pulse transformer having a primary and a secondary winding; a switching means, said switching means being connected in circuit with said second pair of terminals, said iurther.capacitance and said primary winding for sequentially on operation connecting said further capacitance across said second pair of terminals to charge substantially to the voltage of the supply connected thereto, disconnecting said further capacitance from said second pair of terminals without discharging it, and connecting said further capacitance across the primary winding to discharge through it; means connecting the secondary winding across the capacitance in said resonant circuit, the resonant circuit having a decrement and resonant frequency and the spark gap having a break-down potential such that on application of a substantially unidirectional high voltage pulse across the capacitance therein on discharge of said further capacitance through the primary winding when the lamp is connected to the first pair of terminals, said spark gap will break down when the capacitance is 7 charged to a substantial proportion of the peak pulse potential and .a high frequency =disharge will be maintained in the lamp for :at 'least said predetermined minimum time; a third pair of terminals for connection to the direct current :source for use in normal operation of the lamp; :and circuit :means completed on operation of said switching :means i for electrically connecting said third pairof terminals to said .firs'tpair.

"7. .An operating circuit for a high pressure vapor electric discharge lamp adapted to operate normally from a direct current :source and having characteristics :such that on starting operation with any combination of internal temperature and pressure which may occur a high frequency discharge must be maintained between the electrodes of the lamp for a predetermined minimum time before the normal direct current discharge can start and be maintained, said 'circuit comprising: a first pair of terminals for connection to the electrodes of the lamp; a resonant circuit electrically connected across said first pair of terminals; a second pair of terminals for connection to the direct current source for use in normal operationof the lamp; a direct current circuit for normal-operation of the lamp between said-second pair of terminalsand-said first pair of terminals, said circuit including an electrically controlled contact making device for opening and closing the circuit; .a pulse generator for generating substantially unidirectional high voltage pulses; switching means electrically connected to said-second pair of terminals for controlling the said contact making device and the pulse generator by applying energy from the source connected to the-second pair of terminals inoperation, to cause the pulse generator to generatea pulse on operation of the contact makingdevice to close the operating circuit for the lamp; and means for applying the output of said pulse generator across part at least of the resonant circuit, the resonant circuit having a deorement'and resonant frequency such that on application-of a pulse from the pulse generator, when the lamp is connected to the first pair of terminals, a high frequency discharge will be maintained in the lamp for at least said predetermined lninimum time.

8. An operating circuit 'for a high pressure vapor-electric discharge lamp according to claim '7 in which said operating circuit for the lamp includes at least one high frequency choke and relay means having a winding connected in series with the choke in the normal direct current discharge path of the lamp and a contact operated when the current in said winding reaches a value slightly less than the normal discharge currentof the lamp to short circuit said cho'ke.

'9. An operating circuit for a high pressure vapor electric discharge lamp adapted to operate normally from a direct current source and hav- "ing characteristics such that on starting operation with-any combination of internal temperature and pressure which may occur a high frequency discharge must be maintained between the electrodes of the lamp for a predetermined minimum time before the normal direct current discharge can'start and be maintained, said circuit comprising: a first pair of terminals for connection to the electrodes of the lamp; a resonant circuit electrically connected across said first pair of terminals; 2. second pair of terminals for connection to the direct current source for use in normal operationiof the lamp; a direct current i circuit "forlnormal operation of the lamp between said second terminals and said first terminals, said circuit including an electrically controlled contact making device for opening .and closing the circuits; a capacitance and a voltage step up pulse transformer having a primary and a secondary winding; a first switching means; a second switching means electrically connected to said second pairof terminals for controlling the contact making device and said first switching means on operation of the contact making device to close the operating circuit for the lamp, said first switching means being-arranged'on operation sequentially to connect saidcapacitance across said second pair of terminals to charge substantially to the voltage of the source connected thereto in'operation, to disconnect said capacitance from said second pair of terminals without discharging it, and to connect said capacitance across the primary winding of the pulse transformer -to discharge through it; means connecting the secondary winding of the pulse transformer across part at least of the resonant circuit, the resonant circuit having a decrement-and resonant frequency such that on application of a substantially unidirectional high voltage pulse on discharge of said capacitance through the primary winding, when the lamp is connected to the firstpair of terminals, a high frequency discharge will be maintained in the lamp for at least said predetermined minimum time.

10. An operating circuit for a high pressure vapor electric discharge lamp according to claim 9 in which said operating circuit for the lamp includes at least one high frequency'ohoke and relay means having-a winding connectedin series with the choke in the normal direct current discharge path of the lamp and contact operated when the current in said winding reaches a value slightly less than-the normal discharge current-of the lamp to short circuit said choke.

11. An operating circuit for a high pressure vapor electric-discharge lamp according to claim 9 in which said second switching means comprises a-push buttonswitch having a first and a second pair of contacts which are closed on operation in that order-and open in thatorder on release, and afirst relaymeans connected incircuit with said first pair of contacts across said second pair of terminals for-operating said-con- .tact making device so as to close and open the operating circuit for the lamp on alternate occasions of closing said first pair of contacts; and said first switching .means comprises a second relay means and a-changeover switch, said second relay means being connected in circuit with said second pairof contacts across said second pair of terminals and causing the changeover switch to move .from afirst to a second position on .energization and back'on de-energization and said changeover switchin-said first position-connecting the capacitance .across the said second pair of terminals and in said second position connecting the capacitance across the said primar winding.

.12. An operating circuit for a high pressure vapor electric discharge lamp adapted 'to-operate normally .from .a direct current source and having characteristics such that on startingopera- .tion with any-combination of internal temperature and pressure which .may occur-a high frequency discharge must be maintained between the electrodes of the lamp for a predetermined minimum time before the :normal :direct ourrent discharge can start and be maintained, said circuit comprising: a first pair of terminals for connection to the electrodes of the lamp; a resonant circuit electrically connected across said first pair of terminals, said resonant circuit including a capacitance, an inductance and a spark gap connected in series; a second pair of terminals for connection to the direct current source for use in normal operation of the lamp; a direct current circuit for normal operation of the lamp between said second terminals and said first terminals, said circuit including an electrically controlled contact making device for opening and closing the circuit; a capacitance and a voltage step-up pulse transformer having a primary and a secondary winding; a first switching means; a second switching means electrically connected to said second pair of terminals for controlling the contact making device and said first switching means on operation of the contact making device to close the operating circuit for the lamp, said first switching means being arranged on operation sequentially to connect said capacitance across said second pair of terminals to charge substantially to the voltage of the source connected thereto in operation, to disconnect said capacitance from said pair of terminals without discharging it, and to connect said capacitance across the primary winding of the pulse transformer to discharge through it; means connecting the secondary winding across the capacitance in said resonant circuit, the resonant circuit having a decrement and resonant frequency and the spark gap having a break-down potential such that on application of a substantially unidirectional high voltage pulse across the capacitance therein on discharge of said further capacitance through the primary winding when the lamp is connected to the first pair of terminals, said spark gap will break down when the capacitance is charged to a substantial proportion of the peak pulse potential and a high frequency discharge will be maintained in the lamp for at least said predetermined minimum time.

ERIC ARTHUR HOWARD FRENCH.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,955,520 Vawter Apr. 17, 1934 1,961,749 Ewest June 5, 1934 2,001,517 Buttolph May 14, 1935 2,326,597 Abernathy Aug. 10, 1943 2,331,771 Germeshausen et a1. Oct. 12, 1943 2.480.681 Stiefel Aug. 30, 1949 

